In a slow sand filter, the usual water filtration rate is about 100 to 200 litres per square metre per hour (≈0.1–0.2 m/h).

Question

Solve this multiple-choice question and choose the correct option.

Accepted Answer

Introduction / Context:Slow sand filters (SSF) achieve excellent microbiological removal at very low filtration rates thanks to biological processes within the schmutzdecke (biologically active layer). Given Data / Assumptions: SSF are not backwashed; cleaning is by scraping the top sand layer. Rates are an order of magnitude lower than rapid sand filters. Concept / Approach:Design guidance places SSF around 0.1–0.2 m/h, which equals 100–200 L/m2/hr. This supports biologically mediated removal while maintaining effluent quality. Step-by-Step Solution: 1) Recognize that SSF emphasize biological action; high rates would disrupt performance.2) Convert 0.1–0.2 m/h to L/m2/hr: 1 m/h = 1000 L/m2/hr; thus 0.1–0.2 m/h = 100–200 L/m2/hr.3) Choose the option matching this standard range. Verification / Alternative check:Historical SSF designs and WHO guidance cite similar ranges and operational practices. Why Other Options Are Wrong: 10–20: too low; impractically small throughput.1500–2500 or 4000–5000: far too high; these are rapid filtration/pressure filter regimes.600–900: still high for SSF; risks breakthrough. Common Pitfalls:Confusing SSF with RSF; forgetting unit conversions; assuming higher rate is always better. Final Answer:100 to 200 L/m2/hr

In a slow sand filter, the usual water filtration rate is about 100 to 200 litres per square metre per hour (≈0.1–0.2 m/h).

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